1. Field of the Invention
This invention relates to radio-telephone systems, and especially to radio-telephone systems designed for communications between land based and airborne stations. In particular, the invention relates to the channeling and decode circuitry for airborne radio-telephone systems compatible with the selective signalling system prescribed in Sections 22.522 and 22.523 of the Federal Communications Commission Regulations.
2. Description of the Prior Art
The Federal Communications Commission (FCC) has established a nationwide plan for assignment of frequencies to land based stations rendering radio-telephone communications service to airborne stations. The FCC has also prescribed a frequency modulated selective signalling code for individual call-up of airborne stations. Airborne radio-telephone transceivers must be designed to be compatible with the FCC regulations.
The nationwide radio-telephone system for air-to-ground communications includes approximately 100 land based stations, which are interconnected to the nationwide public land line message telephone system and are capable of two-way radio communication with airborne stations. Twelve frequency pairs in the 450 to 460 MHZ range have been designated by the FCC as working channel numbers 1-12 for radio communications between land based stations and airborne stations. Each land based station is assigned one or more working channels, and interference between land stations is reduced or eliminated by assigning like channel numbers only to geographically spaced apart base stations. A common thirteenth channel at 454.675 MHZ is assigned to each base station and is used exclusively as a signalling channel for call-up of airborne stations.
Ground-to-air communications are initiated by selection of a base station in geographical proximity to the estimated position of the aircraft to be called. Each aircraft having an airborne radio-telephone system is assigned an individual five digit call-up number. The base station transmits the five digit number, along with its working channel designation, in pulse coded format on the signalling channel. The signalling channel is continuously monitored by airborne transceivers, and when the correct five digit identification number is decoded by a transceiver, the airborne radio telephone automatically switches to the initiating base station's working channel. An alarm or other display is activated to notify the airborne station operator of the incoming call.
Air-to-ground communications are initiated by selection of a working channel, and transmission from the air station to the land based station on the selected channel. Each land based station continuously transmits a dial tone on its working channel or channels at a low power level to facilitate channel selection. The channel corresponding to the base station transmitting the clearest dial tone is selected by the airborne operator. The land based station shifts to full power transmission when an air station initiated call is received, and the land based station operator connects the airborne station into the land line telephone network.
Selective individual call-up of airborne stations is accomplished by means of a code prescribed by Section 22.522 of the FCC regulations. The code comprises a pulsed, non-return to zero (NRZ) signal modulated on the 454.675 MHZ signalling channel. The code uses frequency shifts between 600 HZ and 1500 HZ to represent numerical digits. The code is comprised of two segments of multiple frequency shifts, representing groups of digits, that embody the address code and working channel number code respectively. A clearing pulse comprising a single frequency shift preceeds the address code group and another follows the channel code group. The first clearing pulse is followed by an idle period of 0.66 to 1.4 seconds. The first segment of multiple frequency shifts follows the idle period and is used to indicate the five digits of an individual airborne station's address code. Succeeding frequency shifts representing a single digit are spaced 0.1 second apart, and interdigital spacing within the address sequence is indicated by time gaps of 0.3 to 0.5 seconds between the last frequency shift of one digit and the first frequency shift of the succeeding digit. The address designation sequence and channel designation sequence of the code are separated by an idle period of at least 1.0 second. The channel designation segment is comprised of two to thirteen frequency shifts spaced 0.1 second apart, representing channels 1 through 12 respectively. The second segment of multiple frequency shifts is followed by an idle period of 0.3 second after which the second clearing pulse comprising a single frequency shift is transmitted. It will be appreciated that a time isolated single frequency shift cannot be used within the address code or the channel designation, since a single frequency shift is reserved as the clearing pulse. In this regard, the transceiver must account for the first frequency shift in each group of multiple frequency shifts so as to count two frequency shifts as one, three shifts as two, etc.
To be compatible with the above-described code, an airborne radio-telephone transceiver must first be able to detect and count the frequency shifts in the received signal. The transceiver must then differentiate between the address designation segment and the channel designation segment, and must be able to differentiate between the frequency shift groups representing digits within the address designation sequence. The selective signalling code or "number" embodied in the code must be compared to the number assigned to the airborne station and the transceiver must have the capability to automatically shift to a code designated working channel upon a proper match of the signalling code and the airborne station's number. The transceiver must be designed to clear its counters to zero each time a time isolated frequency shift is received. The transceiver must also have the capability to select an active working channel when air-to-ground communications are initiated by the airborne station operator.
A circuit capable of accomplishing the above-described functions is disclosed in U.S. Pat. No. 3,859,475, for which I was a co-inventor. The transceiver disclosed in U.S. Pat. No. 3,589,574, however, is relatively bulky, has relatively high power requirements, is dependent on clear transmission and precise reception of both the 600 HZ and 1500 HZ tones, and provides a relatively cumbersome system for selecting a channel when initiating air-to-ground transmissions.